TABLE 4-3 THE BONES OF THE HEAD SKELETON OF THE LIVING 

 AMPHIBIA ARRANGED ACCORDING TO TYPE AND 

 PLACE or ORIGIN 



vomers are broad separate plates bearing teeth. A palatine 

 is lacking. The pterygoid forms much of the palatoquadrate 

 complex; the quadrate is little more than the articular area 

 for the lower jaw— the squamosal supports the palatoquad- 

 rate and attaches it to the cranium through its suture with 

 the otic capsule and parietal. The pterygoid sutures with 

 the broad parasphenoid covering the ventral aspect of the 

 cranium. This is a nonkinetic skull. 



In this group the number of endocranial ossifications is 

 reduced; supraoccipital, basioccipital, basisphenoid, and 

 ethmoid bones are lacking. Cryptobranchus lacks an opis- 

 thotic, as do most amphibians, but this element is present 

 in Necturus. An orbitosphenoid is lacking in Necturus; its 

 place is taken by a process of the frontal. The bilateral oc- 

 cipital condyles are processes of the exoccipitals. The eye is 

 encircled by a sclerotic cartilage. 



The lower jaw is formed of the usual complement of der- 

 mal bones, lacking only the splenial. A coronoid is present 

 (and bears teeth) in Nectunis but not Cryptobranchus. The 

 latter is unique in that it appears to have a supraangular. 

 There are articular and mental ossifications in Meckel's 

 cartilage. 



The hyoid apparatus, or visceral skeleton, is formed from 

 parts of several arches (Figure 4-18). There is a region of 

 ossification in the ceratohyal, the first and second hypo- 

 branchials, and the second and third ceratobranchials of 

 Cryptobranchus. Of the two genera, the branchial skeleton of 

 Cryptobranchus is the better developed; yet it is Necturus which 

 has two pairs of branchial openings, between ceratobran- 

 chials I and II, and II and III, and a large external gill on 

 each of the three ceratobranchials. Cryptobranchus has a 

 single pair of external openings between ceratobranchials 

 III and IV and no external gills. 



The stapes of both has a short stylus and a large footplate 

 set in the fenestra vestibuli of the otic capsule. There are 

 extensive cartilaginous areas above and below the footplate. 

 The stylus is curved toward and sutures with a process from 

 the squamosal in Necturus; there is a ligamentous attach- 

 ment to the same area in Cryptobranchus. This association of 

 stapes and quadrate (through squamosal) is assumed to func- 

 tion in transmission of vibrations (sound) into the inner ear. 



EMBRYOLOGicAL DEVELOPMENT The details of development 

 are not known for either of the genera described but can be 

 inferred from accounts of other salamanders. Chondrification 

 begins in the visceral skeleton with the appearance of the 

 posterior part of the palatoquadrate, Meckel's cartilage, 

 the ceratohyal, and four ceratobranchials with their cop- 

 ulae. In the endocranium the trabeculae shortly appear, well 

 separated — a condition described by the term platytrabic. 

 Each trabecula is continuous with the orbital cartilage of 

 its side through the preoptic, metoptic, and antotic pillars. 

 The posterior end of the trabecula attaches to the para- 

 chordal. The developmental process is partly indicated in 

 Figure 4-19. 



The stapes appears in association with the fissure-like 

 fenestra vestibuli and, as a part of the hyoid arch, is 

 joined to the ceratohyal by a strand of mesenchyme cells 

 which later become attached to the palatoquadrate. 



The dermal bones develop as in higher forms; those 

 which are lacking in the adult are not indicated at anytime 

 in the embryo. On the outside of the quadrate there is 

 a small quadratojugal center which fuses with that bone. 

 The endocranial bones arise as perichondral ossifications, 

 and the exoccipital of Cryptobranchus spreads into the area 

 ordinarily occupied by the opisthotic. The orbitosphenoid 

 arises in the orbital cartilage. The pila antotica does not 

 ossify nor does the ascending process of the palatoquadrate. 



GENERAL OBSERVATIONS The salamanders described do not 

 include all of the variations observed in urodeles. A septo- 

 maxilla, lacrimal, and basiocciphal are present in some. 

 The palatoquadrate complex may pivot on the prootic; that 

 is, the skull may be kinetic. Sclerotic bones occur in at least 

 one salamander, while others lack even the cartilaginous 

 ring. In development the larva may have a horny upper jaw, 

 and an intermandibular cartilage occurs in some. 



80 • HEAD SKELETON OF OTHER TETRAPODS AND CHOANATES 



